A digital amplitude modulator is used for, for example, medium-wave broadcasting. The digital amplitude modulator includes a plurality of power amplifiers. The digital amplitude modulator ON/OFF-controls the plurality of power amplifiers according to the voltage amplitude level of a signal to be modulated, thereby changing the number of power amplifiers to which amplified carrier signals are output. The digital amplitude modulator generates an Amplitude Modulation (AM) wave by synthesizing the output signals of power amplifiers in an ON state, and sends the generated AM wave to a predetermined broadcasting service area.
In the aforementioned digital amplitude modulator, the signal quality of the AM wave may degrade when, for example, a breakdown of the power amplifier or some failure occurs. When an SWR (Standing Wave Ratio) deteriorates at the time of a lightning strike or the occurrence of an abnormal condition in a load system such as disconnection, failure, or mixing of a surge, it is necessary to prevent a breakdown of the power amplifier.
To prevent the power amplifier from a breakdown caused by reflected power from an antenna or the like, the digital amplitude modulator monitors the SWR (Standing Wave Ratio), and controls output power. For example, there is conventionally proposed a method of setting output power to zero by disconnecting the switch of a transmission circuit when the SWR value exceeds a warning limit. There is also known a method of decreasing transmission power when the SWR value exceeds a given value.
Power which actually causes damage to a power amplifier is the sum of loss power occurring in the power amplifier and reflected power consumed by the power amplifier. Even if, therefore, the reflected power becomes equal to or higher than a given value and the SWR exceeds a set value, this does not always cause a breakdown of the power amplifier. Especially when the SWR value becomes large to increase the reflected power, the output power decreases. In this situation, if an operation of protecting the power amplifier is performed by decreasing the transmission power, the coverage area of a broadcasting service area becomes narrow, thereby degrading the signal quality of the AM wave.